Lab #11 Report
The purpose of this lab is to obtain the cooling curves for tin, lead and various alloys of these two metals. A phase diagram will be generated from these cooling curves. When a material is heated by adding energy at a constant rate, the temperature of the material will increase linearly with time, provided a phase transformation does not occur. As temperature of the sample becomes greater than the temperature of the surroundings, the surroundings will also lose heat(energy). This heat loss depends on the temperature difference between the sample and the surroundings, and results in a decrease in the net rate that energy is added to the sample and a slight convexity in the heating curve. When a phase transformation occurs during heating, additional energy is required to form a new phase, thus the rate of temperature increase will slow down during this period of phase transformation (Figure 1). If the sample is pure metal or has a eutectic composition, the phase transformation will occur at a single temperature, resulting in a horizontal region in the heating curve (Figure 2).
Figure 1 - Phase Transformation over a single temperature Figure 2 - Phase Transformation over a region When the sample is cooled by heat loss to the surroundings, the rate of energy loss depends on the sample’s temperature, which results in a concave cooling curve. Phase transformation shows up in the cooling curves in a form similar to that of heating curves. Sometimes super cooling is evident in the cooling curves (Figure 3).
Figure 3 - Cooling Curve
Lead-tin alloys are placed in electrically heated pots, shown in figure 4, with a thermocouple to measure the temperature of each sample. The thermocouples are connected to an Omega DataLogger Thermometer. The DataLogger only has 4 channels. Thus, the experiment has to be performed twice to collect...
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